Pneumatic control unit

ABSTRACT

A precision pneumatic proportional derivative or proportional integral derivative controller having a plurality of pistons each one of which forms a different portion of a balancing beam. Each piston is arranged to pass through a different one of several spaced apart cylindrical passageways formed by inner wall portions of the casing of said controller. A separate rolling diaphragm is connected in fluid tight sealing engagement with each piston and with its associated wall portion to form a rolling seal therebetween. The diameters of these pistons are of predetermined sizes in relation to their associated passageway diameters. As a result, a predetermined difference or offset is allowed to exist between positive and negative feedback forces applied to the balancing beam, thereby allowing the output offset of the controller to deviate from a zero value while it is maintaining its loop gain at a proper value.

United States Patent n91 Akiya ma et al.

1 1] 3,823,726 July 16, 1974 PNEUMATIC CONTROL UNIT [75] Inventors: Tadashi Akiyama; Ryuhei Fikuda; Masanobu Ando; Norikazu Wada, all of Yokohama-City; Toshio Umeda, Tokyo; Tatsuhide Shiga, Zushi-city, all of Japan V [73] Assignee: Honeywell Inc., Minneapolis, Minn.

[22] Filed: June 30, 1972 [211 App]. No.: 268,005

[52] US. Cl. 137/86, 137/84 [51] Int. Cl, G05d 16/00, FlSb 5/00 [58] Field of Search 137/86, 85, 84

[56] References Cited 1 i v UNITED STATES PATENTS 2,712,321 7/1955 Grogan 137/86 3,020,490 2/1962 Kleiss 137/86 X 3,326,228 6/l967 Phillips 137/86 3,394,722 7/1968 Stranahan 137/85 X 3,41 L529 ll/l968 Ba'ssett... 137/85 X 3,465,768 9/1969 Martin 137/86 X Primary ExaminerAlan Cohan Attorney, Agent, or F irm-Arthur H. Swanson; Lockwood D. Burton; John Shaw Stevenson [5 7] ABSTRACT A precision pneumatic proportional derivative or proportional integral derivative controller having a plurality of pistons each one of which forms a different portion of a balancing beam. Each piston is arranged to pass'through a different one of several spaced apart cylindrical passageways formed by inner wall portions of the casing of said controller. A separate rolling diaphragm is connected in fluid tight sealing engagement with each piston and with its associated wall portion to form a rolling seal therebetween. The diameters of these pistons are of predetermined sizes in relation to theirassociated passageway diameters. As a result, a

- predetermined difference or offset is allowed to exist between positive and negative feedback forces applied to the balancing beam, thereby allowing the output offset of the controller to deviate from a zero value whileit is maintaining its loop gain at a proper value.

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260 aim aim n'm UUT ,1 V PNEUMATIC CONTROL UNIT The present invention relates to a pneumatic proportional and integrating (Pl) control unit or a pneumatic proportional integrating and differentiating (PlD) control unit and more particularly to an improved pneumatic type automatic integrating control unit with reset means wherein the output off-set thereof is substantially eliminated by providing a suitable difference between positive and negative feedback quantities.

[t is an object of this invention to provide a pneumatic proportional integrating control unit wherein the output off-set of the pneumatic type automatic control unit with reset means is substantially eliminated.v

Another object of this invention is to provide an improved pneumatic proportional integrating and differentiating control unit in which the output off-set of the pneumatic type automatic control unit with reset means is substantially eliminated.

in accordance with one aspect of this invention, there is provided a pneumatic proportional integrating control unit comprising l a pneumatic operational amplifier including a balancing beam; a casing having a cylin drical opening communicating at its opposite ends with the atmosphere and adapted to receive the balancing beam, and a pair of cylindrical air chambers formed in the casing, the cylindrical air chambers being coaxial with the cylindrical openings; a pair of pressure receiving members, each dividing its respectively associated air chamber into an input pressure receiving chamber and a feedback pressure receiving chamber, the stationary end of each pressure receiving member being sealed to the inner wall of each air chamber and the movable end of the pressure receiving member being sealed to the balancing beam at a predetermined portion thereof; a pair of rolling diaphragms respectively for the input and the feedback chambers, the rolling diaphragms sealing the cylindrical openings of the respective pressure receiving chambers; conduits for transmitting corresponding pressure signals to respective pressure receiving chambers; a nozzle-flapper device for converting the displacement of the balancing beam into an outputair pressure signal having a corresponding magnitude; and a pair of pistons for driving a pair of feedback rolling diaphragrns, the pistons being positioned in the corresponding cylinders, and the outer diameters ofthe pistons being related to the inner diameters of the cylinders so as to provide a predetermined difference between a positive feedback quantity and a negative feedback quantity, (2) a pair of input circuits for transmitting a signal pressure of a control quantity and a signal pressure of a set value to the pair of input pressure receiving chambers, (3) a first feedback circuit for transmitting a negative feedback pressure created by an output pressure to one feedback pressure receiving chamber, and a second feedback circuit for transmitting a positive feedback pressure created by a reset pressure to the other feedback pressure receiving chamber, the second feedback circuit including a reset throttle and a reset capacitor, and (4) a throttle type pressure dividing circuit inserted in either one or both of the pair of input circuits and the pair of feedback circuits for adjusting the proportional band.

According to another aspect of this invention there is provided a pneumatic proportional integrating control unit comprising l a pneumatic operational amplifier including a pair of input pressure receiving chambers, a pair of feedback pressure receiving chambers, a plurality of pressure receiving members, each partitioning each pair of the pressure receiving chamber, a balancing beam connected to the pressure receiving members to be displaced thereby, and a nozzle-flapper device for generating a pneumatic pressure signal corresponding to the displacement of the balancing beam, the pneumatic operational amplifier providing a predetermined difference between a negative feedback quantity created by an output pressure and a positive feedback quantity created by a reset pressure, (2) a first feedback circuit for. transmitting a negative feedback pressure to the first'feedback pressure receiving chamber of the pneumatic operational amplifier, and a second feedback circuit for transmitting a positive feedback pressure acting as a reset pressure to the second feedback pressure receiving circuit of the pneumatic operational amplifier circuit, (3) a pressure dividing circuit interposed between the first and second feedback circuits for adjusting the proportional band, (4) a pneumatic 1:1 relay included in the second feedback circuit for preventing air leakage to the proportional band adjusting circuit and for transmitting the reset pressure to the second feedback pressure receiving chamber at a ratio of l :1, (5) a compensating pneumatic 1:1 relay included in the first feedback circuit for compensating the output off-set of the first mentioned pneumatic 1:1 relay, (6) a pair of input circuits respectively transmitting to the pair of input pressure receiving chambers a control quantity signal pressure and a set value signal pressure and, (7) an output circuit for deriving an output signal pressure out of the control unit.

According to still another aspect of this invention, there is provided a pneumatic proportional integrating control unit comprising, (1) a pneumatic operational amplifier including a pair of input pressure receiving chambers, a pair of feedback pressure receiving chambers, a plurality of input pressure receiving members, each partitioning each pair of the pressure receiving chambers, a balancing beam connected to the pressure receiving members to be displaced thereby, and a nozzle-flapper device for producing an air pressure signal corresponding to the displacement of the balancing beam, the pneumatic operational amplifier providing a difference of a definite quantity between a negative feedback quantity created by an output pressure and a positive feedback quantity created by a reset pressure, (2) a first feedback circuit, for transmitting a negative feedback pressure to the first feedback pressure receiving chamber of the pneumatic operational amplifier, and a second feedback circuit for transmitting a positive feedback pressure to the second feedback pressure receiving chamber of the pneumatic operational amplifier, (3) a pair of input circuits respectively transmitting a control quantity signal pressure and a set value signal pressure to said pair of input pressure receiving chambers of the pneumatic operational amplifier, (4) a pressure dividing circuit connected between the pair of input circuits for adjusting the proportional band, and (5) an output circuit for deriving an output signal pressure out of the pneumatic control system.

Further objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a diagrammatic connection diagram of a pneumatic control apparatus embodying the invention;

FIG. 2a shows a longitudinal section of an opera tional amplifier unit utilized in this invention;

FIG. 2b shows a longitudinal section of an assembly incorporating the operational amplifier unit;

FIG. 20 is a partial bottom view of the assembly shown in FIG. 2b;

FIG. 3 shows a longitudinal section of a switch;

FIG. 4 is a diagram showing a pneumatic I:l relay;

FIG. 5 shows a graph adapted to explain the operation of the pneumatic l:l relay;

FIG. 6 is a diagram showing a rate unit;

FIG. 7 shows a longitudinal section of the rate unit;

FIG. 8 is a perspective view showing an orifice throttle and an orifice plate utilized in the rate unit shown in FIG. 7;

F IG. 9 is a graph showing the input-output characteristic of the rate unit shown in FIG. 7;

FIG. 10 is a longitudinal sectionalview of one example of a pneumatic pilot valve;

FIG. 11 shows an electrical equivalent circuit and a characteristic of a pressure dividing circuit utilizing screw throttles or restrictions;

FIGS. 12, 13, 14, 15 and 16 show block diagrams for explaining the operation of this invention, and

FIG. 17 is a Bode diagram.

Referring first to FIG. 1 showing an example of the PID pneumatic control apparatus embodying the invention, the PID pneumatic control apparatus shown therein comprises a pneumatic operational amplifier unit 45 including a pair of air inlet ports 86 and 87, a pair of feedback air inlet ports 88 and 89, and a nozzle flapper mechanism constituted by a flapper 68 and a nozzle 69 and functions to convert the displacement of a balancing beam into a corresponding output air pres sure; a screw throttle R a cylindrical fixed restriction R and a transfer switch SW which constitutes proportional band adjusting means on the input side; restrictions R and R, which constitute proportional band adjusting means on the input side; a reset restriction R and a reset capacitor C, which constitute a reset unit; a rate unit 46; a first 111 pneumatic relay 41 which functions to transmit the reset pressure to the pneumatic amplifier unit 45 at a ratio of one to one and to prevent the leakage of the air pressure from the reset unit to the proportional band adjusting means on the feedback side; a second 111 pneumatic relay 42 included in the negative feedback circuit for the purposeof compensating the characteristic of the first pneumatic relay 4] and preventing the leakage of the air pressure from the rate unit to the proportional band adjusting means on the feedback side; a rate-off switch 37, a pilot valve 48; a manual control unit 49 which functions to generate a desired manual operating pressure and to follow-up the automatic operating pressure; a balancing unit 47 for providing the manual-automatic switching operation without a bump and without the necessity of any balancing operation; a delay switch SW for providing the bumpless manual-automatic switching operation for the PID control apparatus; automatic-manual transfer switches SW and SW a manually operated automatic-manual transfer switch SW,; a manually operated transfer switch SW 4 interlocked with the manually operated automaticmanual transfer switch SW, for transmitting the operating pressure to the transfer switches SW SW and SW,, at the time of automatic operation; and air conduits 1, 2, 3 and so forth interconnecting above described elements. There are also provided a positive negative transfer switch D-R for the input air pressure, a cylindrical fixed restriction R inserted in conduit 6 provided for transmitting the supply air pressure FAS to nozzle 69 and a cylindrical fixed restriction R associated with the relay switch SW As diagrammatically shown in FIG. 6 the rate unit 46 comprises a bellows unit 38, a rate throttle R and a rate capacitor C Like the reset throttle R rate throttle R takes the form of a nozzle-flapper type variable throttle. As shown in FIG. 1, the rate throttle R comprises a casing 144 containing a nozzle 143, a flapper 145 fulcrumed by a shoulder of the nozzle, a leaf spring 146.for biasing the flapper and means for adjusting the inclination of the flapper to adjust the degree of throttling. The casing 144 is provided with air inlet ports 141 and 142. Means for adjusting the inclination of the flapper, that is the degree of throttling comprises an eccentric cam 147, an operating knob for the eccentric cam, not shown, a cam follower 149 and an adjustable screw for urging one end of flapper 145 against cam follower 149.

The l:l pneumatic relays 41 and 42 have the same construction. As diagrammatically shown in FIG. 4, the relay comprises a casing 120, a diaphragm 125 dividing the interior of the casing into two air chambers 128 and 129, and a nozzle shaped discharge port 126 opening into chamber 128. An air pressure supply conduit 122 is communicated with chamber 128 through a restriction 123, whereas output pressure conduit 124 is communicated directly with the chamber 128. Another input pressure conduit 121 is connected directly to the other air chamber 129.

As shown in FIG. 2a and FIG. 2b, the pneumatic operational amplifier unit 45 comprises a casing 57, containing a pair of input pressure receiving chambers 51 and 52, a pair of feedback pressure receiving chambers 53 and 54, an input pressure receiving member and a feedback pressure receiving member 76, both in the form of bellows, and a balancing beam 67 connected between the movable ends of the pressure receiving members to be displaceable in response to the received air pressure signals. Within casing 57 are contained two pairs of cylinders 81, 82 and 83, 84 which are aligned on a straight line and arranged coaxialy with the balancing beam 67. These cylinders are communicated with the atmosphere at their one ends. Cyplinders 82 and 83 are formed integral and are vented to the atmo sphere through a vent passage 55. These cylinders have the same inner diameter. Provision of the cylinders communicates all pressure receiving chambers with the atmosphere so that it is necessary to seal these chambers against the atmosphere. To this end rolling diaphragms 71, 72, 73 and 74 are provided. Each rolling diaphragm is mounted by securing its movable end to the balancing beam 67 and its stationary end to one end of the cylinder. Respective cylinders contain pistons 61, 62, 63 and 64, respectively. Respective pistons are secured to balancing beam 67 such that, at the zero position of the balancing beam, the heads of the pistons coincides with the ends of the corresponding cylinders phragms.

Proper selection of the outer diameter of the piston is essential to the improvement of the off-set. More particularly, as shown exaggerated in FIG. 2a, the outer diameter d, of piston 64 is smaller than that a, ofopposed piston 63. Another pair of opposed pistons 61 and 63 have the same outer diameter d, which is the same as that of piston 63. For this reason, it is advantageous to form pistons 62 and 63 as an integral unit to be received in integral cylinders 82 and 83.

The purpose ofthe balancing beam 67 is to derive the resultant displacement of a pair of pressure receiving elements 75 and 76. Actually however, it is advantageous to use a balancing beam assembly shown in FIG. 2b for the purpose of utilizing rolling diaphragms driven by pistons to provide the seals and to differentiate the piston diameters in order to improve the characteristic of the apparatus. The balancing beam assembly shown in FIG. 2b comprises a balancing beam 67, pistons 61, 62, 63 and 64, and piston type bellows mountings 65 and 66. Piston 74 is welded to the rear surface of the balancing beam 67. Then diaphragm 74, bellow mounting 66, diaphragm 73, pistons 63 and 62, diaphragm 72, bellow mounting 65 and diaphragm 71 are mounted on the balancing beam in the order mentioned and finally piston 61 is bolted to the balancing beam 67 for fastening the movable ends of respective rolling diaphragms and the movable ends of the bellows thus completing the balancing beam assembly.

As shown in FIG. 2b, the balancing beam assembly is biased in the upward and downward directions within the casing of the operational amplifier unit 45 by means ofcoil springs 96 and 92. The lower coil spring 92 is engaged by a zero adjusting member 93 having an operating knob 94.

As shown in FIG. 1, the screw throttle R, acting as the proportional band adjusting means on the input side utilizes a small gap between the male and female screw threads (usually such a gap is not desirable) as the throttle for the passage. of air. The screw throttle R, comprises a casing 101 formed with a female screw 102 between an inlet chamber106 and an outlet chamber 107 and a male screw 103 having screw threads of the same diameter and pitch as the female screw, an O-ring being provided around the head of the male screw. Accordingly, the screw throttle R, can be manufactured very easily and can be adjusted readily. It is important to note that the screw throttle greatly contributes to the improvement of the characteristic of the apparatus. More particularly, it was found that a pressure dividing circuit including a series screw thread R, anda parallel cylindrical throttle R as shown in FIG. 11 has an excellent operating characteristic. It was also confirmed by experiment that the operating characteristic of such a pressure dividing circuit is nearly straight when it is set to provide a ratio of pressure division of :1. Although the proportional band adjusting circuit on the input side is constituted by a pressure diving circuit including throttles R, and R actually the circuit operates either to not divide the input pressure IPV-SPI or divide it at a ratio of 10:1, so that it is possible to adjust in the factory the throttle R, so as to provide the desired pressure division when it is combined with fixed throttle R and then fix the adjusting member by means of a synthetic resin, for example, thus making it impossible to adjust the throttle R, in the field. In this manner, the proportional band on the input side is set to only one ratio 1201 and the transfer switch SW utilized to provide this ratio is constructed to be free from any leakage as diagrammatically shown in FIG. 3. Since this switch SW is constructed substantially identically to automatic-manual transfer switches SW,, SW, and SW, except that it can be operated manually, its description is believed unnecessary.

The proportional band adjusting circuit on the feedback side is constituted by a pressure division circuit including a series throttle R and a parallel throttle R,, as shown in FIG. 1. These throttles are required to have the same characteristic. In this embodiment, both throttles take the form of needle throttle valves in which throttle R, is fixed and throttle R is variable. For this purpose throttle R is provided with an adjusting knob cooperating with a suitable scale, not shown. Utilization of throttles of the same characteristic to form the pressure dividing circuit is advantageous to improve the linearity. We have confirmed, both theoretically and experimentally, that throttles of the same characteristic provide a pressure division circuit of excellent linearity. The proportion band adjusting circuit on the feedback circuit having such an excellent linearity that causes the operational amplifier unit to attain the desired object of improving the off-set. Above described l:l pneumatic relays 41 and 42 also contribute to this object.

As above described each of the 1:1 pneumatic relay 41 and 42 hasthe construction shown in FIG. 4. However, such a 1:1 pneumatic relay inherently accompanies an output off-set so that its input-output characteristic deviates from an ideal 1:1 characteristic line A to line B shown in FIG. 5. Such deviation is caused by the construction of the pneumatic relay. For this reason, it is extremely difficult to eliminate the off-set with a single pneumatic relay. More particularly the pneumatic relay has an air supply port 122 and'an air outlet port 126 which is faced to one side of diaphragm 125. Denoting the outer diameter of the outlet port 126 by X, and the inner diameter by X the effective area one side of the diaphragm which faces air chamber 128 decreases by about 1r/4(X, X /2) This decrease shifts the 1:1 characteristic A to B, as shown in FIG. 5.

It is important to connect the 1:1 pneumatic relay 41 in the reset circuit such that its air chamber 129 (FIG. 4) is connected to reset capacitor C, in order to prevent air leakage from the reset circuit to the proportional band adjusting circuit on the feedback side. If there is some cause that results in an error in the feedback circuit, the off-set eliminating function of the improved pneumatic operational amplifier unit 45 utilized in this circuit would be nullified. According to this invention, to oppose 1:1 relay 41 included in the positive reset circuit, is included in the negative feedback circuit 1:1 pneumatic relay 42 having the same characteristic thus completely eliminating the output offset from the differential signal between positive and negative feedback circuits. It is also possible to use 1:1 pneumatic relay 41 in a control apparatus including a rate unit for the purpose of preventing air leakage from the rate circuit to the proportional band adjusting circuit on the feedback circuit.

FIG. 7 shows a longitudinal sectional view of the balancing unit 47' which comprises a pressure receiving section and a unique nozzle-flapper section. The nozzle-flapper section shown in the lower portion of FIG.

7 comprises an orifice plate 177 including an orifice opening 178, means for deriving out the nozzle back pressure P from a point on the downstream side of nozzle opening 178, a clamping member 176, a nozzle 161 on one end of the clamping member, and means for admitting inlet air pressure. The orifice plate 177 is shown on the lower side of FIG. 8, whereas clamping member 176 is shown on the upper side. Clamping member 176 has a central bore 181 and radial slots 183 are formed through the inner end 182 of the clamping member 176 for deriving the back pressure. A plug 175 for supporting the orifice is provided with inlet air passages 191 and 196 and a passage 192 for deriving out the nozzle back pressure, the communication between passages 191 and 192 being interrupted by an O-ring 196. The plug 175 is coupled to a base block 151 through screw threads 193 and 194. The base block 151 is formed with a passage 171 leading to a nozzle 161 and an axial chamber 185 which is sealed against the atmosphere by means of a metal ball 186. The seal between the chamber 185 and the passage 192 for deriving out the nozzle back pressure is provided by the engagement at 197 between base block 151 and plug 175.

The pressure receiving section comprises a pair of pressure receiving chambers 154 and 155, a pair of pressure receiving members 156 and 157 such as diaphragms for defining the pressure receiving chambers, and a member 158 contacting the pressure receiving members for transmitting the displacement thereof to a flapper 160. The left hand end of flapper 160 is connected to block 151 through a leaf spring 162 and a screw 163. Flapper 162 is suitably biased by means of a pair of coil springs 164 and an adjusting member 165. Clamps 159 are used to clamp diaphragms 156 and 157 in position and the operating air is supplied to chambers I54 and 157 through passages 152 and 153 in block 151. The pair of pressure receiving chambers have substantially the same configuration so as to equalize the effective areas of respective pressure receiving elements. Accordingly, the vertical displacement of flapper 160 is proportional to the difference between two received air pressure signals P, and P and the direction of the displacement is determined by the sign of the difference (P, P In the embodiment.

shown in FIG. 7, if P, P the flapper 160 is displaced upwardly. On the other hand, if P, P the flapper is displaced downwardly. Expressing the displacement of the flapper in terms of the distance X (mm) from nozzle 161, the pneumatic output signal P (kg/cm) of the balancing unit 47 will vary as shown by FIG. 9 wherein the abscissa represents the distance X (mm) between the nozzle and the flapper while the ordinate the magnitude of the output air pressure signal P (kg/cm P, represents the input air pressure which is supplied from a source ofclean air of 1.4 kg/cm", for example, not shown, and the origin 0 represents the atmospheric pressure. As can be noted from the characteristic curve shown in FIG. 9, the nozzle-flapper used in this invention can produce an output air pressure signal P lower than the atmospheric pressure so that it is characterized by a high gain and excellent linearity in the operating range. These features are attributable to the specific construction of the nozzle wherein an orifice opening 178 is used as the nozzle throttle and the decrease in the air pressure caused by the eddy current of air created near the orifice opening is derived out as the output air pressure signal by forming the passage 192 for the output air pressure signal close to the orifice opening. In addition to this advantageous feature, this construction enables to select any value of the distance between the nozzle 161 and the orifice opening 178. We have confirmed by experiment that the distance between the nozzle and the orifice opening does never affect the output characteristic.

FIG. 10 shows one example ofa pilot valve including a casing 200, a first diaphragm 209, a second dia phragm 210, coupling means 213 for assuring said two diaphragms as an integral body to vibrate, a first valve seat 211 formed in said coupling means 213, a second valve seat 214 fixed to the casing 200, a poppet valve assembly 220 having a valve 212 responsive to the first valve seat 211 and the other valve 215 responsive to the second valve seat 214, a coiled spring 217 applying said poppet valve 220 with an upward biasing force, the other coiled spring 216 applying the first and second diaphragms with an upward biasing force and a clamper or bolt 219. Numerial orders 201, 202, 203 and 204 represent a pressure receiving port, an output pressure port, an air inlet port and an air discharge port, respectively. In application of this pilot valve to the pneumatic control apparatus shown in FIG. 1, air conduits 13 and 14 are connected to the pressure receiving port 201 and the output pressure port 202 set forth, respectively.

The control apparatus constructed as above described operates as follows: In FIG. 2a, symbols SP, PV, OUT and RESET represent respectively the air pressure signals of the set quantity, controlled quantity,

output quantity and reset quantity. Under the steady state, it is possible to analyze the operation by neglecting the reset throttle R and capacitor C,. The block di agram of a well known prior art pneumatic type automatic control unit with reset means is shown by FIG. 12 where A,, A B and B represent the constants of PV, SP, OUT and RESET, respectively. Ordinarily, it is designed to satisfy the relations A =A =A and B,=B =B. G represents the forward gain of the feedback circuit. FIG. 13 also shows a block diagram of a conventional prior art pneumatic type automatic control apparatus with reset means showing a simplified form of FIG. 12 for the case wherein A=B by assuming 21 I00 percent proportional band for the sake of explanation.

Under the steady state, if the output OUT varies I00 percent when the deviation PV-SP changes a percent, the quantity a is generally defined as the output off-set of the pneumatic control system and IOO/(i as the integration gain. As can be noted from the block diagram shown in FIG. 13, in the conventional pneumatic type automatic control apparatus with reset means, the output off-set is represented by l/AG while the integration gain by AG as is illustrated in FIG. 12. Accordingly, in the prior art pneumatic control apparatus, it is clear that, in order to decrease the output off-set, it is necessary to increase the constant A which is representative of SP and PV and the forward gain G of the feedback system. However, from the standpoint of design, as it is difficult to increase constant A too much it has become the usual practice to increase the forward gain G. However, increase in the gain G results in the increase of the loop gain BG at the time of proportional control thus causing not only a hunting of the system but also an increase of the cost, thereby nullifying the advantage of decreasing the off-set. For this reason, in the conventional pneumatic control apparatus, thereeis a limit for the improvement of the off-set so that it has been the common practice to select a suitable value of the loop gain G.

The invention contemplates the provision of an improved pneumatic control apparatus capable of reducing the output off-set substantially to zero while maintaining the loop gain BG at a proper value. Thus, according to this invention, the output off-set can be decreased substantially to zero by differentiating the feed back quantities while maintaining the loop gain BG at a proper value. In the illustrated example, although the actual output of the control apparatus is the output from the pilot valve PILOT, since the pilot valve PILOT as shown in FIG. operates as an amplifier, the output value OUT of the control apparatus is proportional to the feedback value FB. Accordingly, in the following description it is assumed that the gain of the pilot valve PILOT or the amplifier equals 111 and that FB=OUT.

Describing the operation of the control apparatus with reference to FIGS. 2a and 12, by using above described constants B B and B, under the steady state, the quantity off the negative feedback value OUT is shown by B, and the quantity of the positive feedback value RESET by B The desired difference is provided by making B B and the pneumatic control apparatus is designed to satisfy the following relations According to this invention, these relations are satisfied by the construction shown in FIG. 2b, a portion thereof being shown exaggerated in FIG. 2a.

As shown in FIG. 2a, different from the prior art control apparatus two pressure receiving members are provided in this invention, one on the input side and other on the feedback side. According to the prior art design, two pressure receiving members are required respectively on the input and feedback sides. However, it is difficult to select four pressure receiving elements having the same operating characteristic so that the difference between their effective areas adversely effects the operating characteristic of the pneumatic control apparatus. In contrast, in accordance with this invention, since only one pressure receiving element is used on the input side as well as on the feedback side, it is not necessary to consider the difference in the effective areas of the pressure receiving members. However, even with this improved construction it is necessary to solve the problem of sealing against the atmospheric pressure. According to this invention, this problem can also be solved readily by. the adoption of a rolling diaphram. By experiment, we have confirmed that the effective area of the rolling diaphram can be maintained at a constant value regardless of the variation in the displacement and pressure, provided that the diameters of the piston and cylinder are maintained constant. As shown in FIGS. 2a and 2b, the effective area of the rolling diaphram is much smaller than that of the bellows acting as the pressure receiving elements. A pair of rolling diaphrams are provided on both input and feedback sides for providing seals so that the difference in the effective areas between opposing diaphrams should be considered. Fortunately, it is possible to manufacture the pistons and cylinders and to fabricate the rolling diaphragms so as to provide a difference between effective areas ofa value of about 0.5 to 1.0 percent. Moreover, as above described, the effective area of the rolling diaphrams is considerably smaller than that of the bellows. Assuming a ratio of 10:1 the difference between the effective arcas ofopposed diaphragm: is very small of the order of about l/lO (0.5 l.0) =0.05 to 0.1 percent, when compared with that between the effective areas of the bellows under the same applied pressure. In this manner, the problem caused by the difference in the effective areas can be efficiently solved by using a pair of rolling diaphrams in each pressure receiving chamber for providing seals. Thus, the provision of the seals does not increase the output off-set.

The construction required for the substantial improvement in the off-set is related to the seals. In other words, according to this invention, the difference in the feedback quantity shown by equations (I) AND (2) is provided by the unique construction of the seals. As shown exaggerated in FIG. 2a, in order to solve the problem of the difference in the effective areas of the pressure receiving members, according to this invention, a single feedback pressure receiving element 76 is used to define two feedback pressure receiving chambers. In addition, a pair of rolling diaphrams having the same characteristic but having a proper difference between their effective areas are used to form seals for the pair of feedback chambers. As the effective area of the rolling diaphragm is expressed by the area of an circle having a diameter equal to that of the movable end of the rolling diaphragm, the effective areas of rolling diaphragms 73 and 74 are shown by following equations.

S 7T/4(D (1 /2) where D represents the inner diameter of the cylinder and d and d outer diameters of pistons 63 and 64, respectively.

The effective area S of a bellows is shown by the area of a circle having a diameter equal to one half of the sum of its inner diameter and outer diameter. Under the steady state of the improved automatic control unit shown in FIG. 2a, calculation of the force applied upon the balancing beam 67, through the pressure receiving member or bellows 76, by the feedback air pressure signal OUT (kg/cm and RESET (kg/cm prevailing in the feedback chamber is described below.

The bellows 76 having an effective area 8,, (cm is applied with RESET (kg/cm prevailing inside the bellows 76 and in turn applies a downward force of RESET X S (kg) upon the balancing beam 67, whereas the diaphragm 74 having an effective area s (cm applies the balancing beam 67 with an upward force of RESET X s (kg). Similarly, the other air pressure, that is the feedback air pressure OUT (kg/cm causes simultaneously an upward force of OUT X S (kg) and a downward force of OUT X s, (kg) on the balancing beam 67. Considering relations S s and S s the balancing beam 67 is applied with an upward force of OUT X (S s (kg) due to the feedback air pressure signal OUT (kg/m as well as a downward force of RESET (S s (kg) due to the air pressure signal RESET (kglcm Let S, and S, be defined by the following equations.

' piston 63 is larger than the outer diameter d of piston 64. For this reason, the effective areas of rolling diaphrams 7'3 and 74 have a relation S, S Accordingly, upward and downward forces acting upon bellows 76 have a relation S, S Denoting the difference between S, and S, by As and assuming a relation S,=S then S =S+As. The relation between As andS is determined as follows.

AS/S NBC .9).

where BG represents the loop gain described above. Equation (7) can be rewritten as follows S is equal to the force S, which is applied upon bellows 76 by the negative feedback air pressure signal OUT whereas S As is equal to the force caused by the positive feedback air pressure signal RESET so that equation (8) represents the ratio between two forces, that is the ratio between two feedback quantities B, and 8,, thus showing that equations (1) and (2) are satisfied. As can be understood from equations (3) (4) (5) and (6), as it is possible to determine the values of S and As by the suitable design of the dimensions and configurations of the bellows, rolling diaphragms, cylinders and pistons, it is easy to design to satisfy equation (7).

In a preferred embodiment of this invention, it was designed: d, 6.45 mm, d 7.00 mm, D 10.00 mm, s 6.7 cm and Ask- 0.57 percent. Because in this em bodiment, a value of the loop gain l/O.57 X 0.01 I75 was used.

Since, according to this invention, the feedback quantity on the reset side is made larger than the negative feedback quantity by the reciprocal of the loop gain BG, under the steady state, it is possible to represent the control system by an equivalent block diagram shown in FIG. 16 in which OUT PV-SPT Since the denominator of equation (9) equals zero its answer becomes infinity. Consequently, the output offset becomes zero and the integration gain becomes infinity.

The dynamic characteristic of the novel control system is also excellent. As above described, FIG. 12

shows a block diagram of a conventional pneumatic type automatic control apparatus with reset means under the steady state. An ideal primary system can be obtained by adding throttle R and reset capacitor C, to the feedback side. The ideal system is expressed by the following equation RESET/OUT l/l T, where T represents the time constant of the integrating throttle and s the operator of the Laplace transformation.

Neglecting another characteristics, let us determine as follows the constants A,. A,. B, and B of SI. I\'. OUT and RESET.

Then, the pneumatic control apparatus of this invention canbe shown by a block diagram shown in FIG. 15 and the ratio between PV-SP and OUT, that is the transfer function is shown by equation (12).

When there is no difference in the feedback quantities, that is in the conventional pneumatic control apparatus, the transfer function corresponding to equation (12) is expressed by equation (13) and the block diagram thereof is shown by FIG. 14.

OUT AG T,+1 Pv sP 1+BG v(12+ 1 1+BG) I (13) It is noted that the dynamic characteristic of inputoutput relation of the improved pneumatic control apparatus involves an integration term l/T, contained in second term on right side of the equation (12).

The Bode diagram corresponding to equation I2) is shown by the solid line of FIG. 17 whereas equation (13) is shown by the dotted line. As can be clearly noted from FIG. 17, since the control apparatus of this invention does not saturate even at low frequencies it is possible to improve the output off-set over the prior art apparatus which saturates at low frequencies.

Above description was made on the assumption that there is no cause of the off-set in the feedback circuit. In this invention, this assumption holds truebecause the off-set is compensated for by the l :1 pneumatic relay. More particularly, since the throttle circuit is provided for the feedback circuit for adjusting the proportional band, the feedback circuit of this invention re-' quires the use the 1:1 pneumatic relay which is not necessary for the circuit arrangement shown in FIG. 2a. However, as shown in FIG. 5, the 1:1 pneumatic relay is designed to have the characteristic B which is displaced a little from the ideal characteristic A. Accordingly, a compensating relay 42 is included in the negative feedback circuit to apply both OUT and RESET to corresponding feedback pressure receiving chambers after transforming them according to the characteristic which is slightly shifted from the ideal characteristic. These two feedback pressures act differentially upon a single feedback pressure receiving member. Thus, the off-set of the relay is eliminated by the differential pressure, thus providing the desired signal. This elimination is attained by the characteristic construction of the pneumatic operational amplifier unit shown in FIGS. 2a and 2b.

As above described, according to this invention, the

output off-set of a pneumatic PI control or a pneumatic PlD control apparatus is substantially eliminated by providing a suitable difference between the positive and negative feedback quantities, that is by using a novel pneumatic operational amplifier unit. More particularly, the off-set caused by the difference in the effective areas of the pressure receiving members is eliminated by using a single bellows. Further, the problem of the seal caused by the use ofa single bellows and the problem caused by the difference in the effective areas of a pair of rolling diaphragms used to form the seals are solved by the proper selection of the preferred characteristic of the rolling diaphragms and by the prope'r selection of the dimensions of the cylinders and pistons which cooperate with the rolling diaphragms. Further, the required difference in the feedback quantities is provided by equalizing the diameter of a pair of opposed cylinders and by providing a suitable difference between the outer diameters of pistons 63 and 64 disposed in the cylinders for driving the rolling diaphragms. Alternatively, it is also possible to differentiate the inner diameters of two cylinders and to make same the outer diameters of two pistons. Further, it is also possible to differentiate the outer diameters of two cylinders or the inner diameters of two pistons. In short, it is the feature of this invention to provide a difference between the effective areas of a pair of rolling diaphragms having substantially the same characteristie and dimension by properly designing the diameters of the cylinders and pistons.

ln the pneumatic control apparatus wherein a proportional band adjusting circuit is inserted on the feedback side it is necessary to eliminate the cause in the feedback circuit that causes the off-set. According to this invention, the off-set is efficiently eliminated by compensating the output off-set of 1:1 pneumatic relay 41 by the 1:1 pneumatic relay 42 having substantially the same characteristic.

What we claim is:

ciated with each of said passageways, a balancing beam extending through said passageways in alignment with the axis of said housing and arranged for axial movement with said pistons in their associated passageways, the diameter of the piston in one end of said housing being smaller than the diameter of the other pistons. a separate rolling diaphragm associated with each ofsaid pistons that are at the opposite ends of said housing for sealing the associated passageways, a separate rolling diaphragm associated with each of the opposite ends of said third piston for sealing the opposite ends of said intermediate passageway, a first bellows providing a seal with said balancing beam at its closed end at a position intermediate a first end one of said pistori s arid the intermediate piston and providing a seal at its open end with an associated internal wall of said housing, a second bellows providing a seal with said balancing beam at its closed end at a position intermediate the other end one of said pistons and said intermediate piston and providing a seal at its open end with an associated internal wall of said housing, each of said bel- .lows forming the common wall of adjacent chambers,

a conduit connection through the wall of said housing to each of said chambers, means to apply through said conduit connections to the opposite sides of the common wall formed by said first bellows setpoint and process variable fluid pressures, means to apply through one of tqhe remaining conduit connections that is associated with the chamber having said smallest piston a positive feedback fluid pressure, means to apply to the other one of said remaining conduits a negative feedback fluid pressure, said negative feedback and positive feedback fluid pressures being applied to the opposite sides of the common wall formed by said second bellows, said last mentioned bellows being associated with said piston of smaller diameter, a nozzle flapper device associated with said balancing beam for converting displacement of said beam into an output fluid pressure having a magnitude corresponding to the magnitude of such displacement, and means to apply said output fluid pressure to the opposite sides of said second bellows, the diameter of the piston in the positive feedback chamber end of the housing with which said second mentioned bellows is associated being of said smaller diameter than said other pistons whereby improvement is obtained in the output offset of said controller without affecting the value of the loop gain of said controller. 

1. A pneumatic control unit, comprising a cylindrical housing having first and second passageways at opposite end portions thereof and a third passageway intermediate said first and second passageways, all of said passageways being of the same size and in alignment with the axis of said cylindrical housing, a piston associated with each of said passageways, a balancing beam extending through said passageways in alignment with the axis of said housing and arranged for axial movement with said pistons in their associated passageways, the diameter of the piston in one end of said housing being smaller than the diameter of the other pistons, a separate rolling diaphragm associated with each of said pistons that are at the opposite ends of said housing for sealing the associated passageways, a separate rolling diaphragm associated with each of the opposite ends of said third piston for sealing the opposite ends of said intermediate passageway, a first bellows providing a seal with said balancing beam at its closed end and a position intermediate a first end one of said pistons and the intermediate piston and providing a seal at its open end with an associated internal wall of said housing, a second bellows providing a seal with said balancing beam at its closed end at a position intermediate the other end one of said pistons and said intermediate Piston and providing a seal at its open end with an associated internal wall of said housing, each of said bellows forming the common wall of adjacent chambers, a conduit connection through the wall of said housing to each of said chambers, means to apply through said conduit connections to the opposite sides of the common wall formed by said first bellows setpoint and process variable fluid pressures, means to apply through one of tqhe remaining conduit connections that is associated with the chamber having said smallest piston a positive feedback fluid pressure, means to apply to the other one of said remaining conduits a negative feedback fluid pressure, said negative feedback and positive feedback fluid pressures being applied to the opposite sides of the common wall formed by said second bellows, said last mentioned bellows being associated with said piston of smaller diameter, a nozzle flapper device associated with said balancing beam for converting displacement of said beam into an output fluid pressure having a magnitude corresponding to the magnitude of such displacement, and means to apply said output fluid pressure to the opposite sides of said second bellows, the diameter of the piston in the positive feedback chamber end of the housing with which said second mentioned bellows is associated being of said smaller diameter than said other pistons whereby improvement is obtained in the output offset of said controller without affecting the value of the loop gain of said controller. 